NANO LETTERS
Ab Initio Study of Doped Carbon Nanotube Sensors
2003 Vol. 3, No. 4 513-517
Shu Peng* and Kyeongjae Cho Department of Mechanical Engineering, Stanford UniVersity, Stanford, California 94305 Received February 4, 2003; Revised Manuscript Received February 27, 2003
ABSTRACT Recently great advances have been made in demonstrating the viability of using carbon nanotubes (CNTs) to detect the presence of chemical gases such as NO2, NH3, and O2, and they have led to the design of a new breed of sensor devices. Based on intrinsic CNTs, the devices are capable of detecting small concentration of molecules with high sensitivity under ambient conditions. However, these devices have a limitation that only molecules binding to a carbon nanotube can be detected. They are currently limited to NH3, NO2, and O2, and a host of highly toxic gases (such as carbon monoxide), water molecules, and biomolecules cannot be detected using these intrinsic CNT devices. Recent efforts on externally functionalizing CNT surface and internal doping in CNT only result in temporary sensing capability due to the weak van der Waals interaction between CNT and doped materials. In this paper, we propose the concept of a new type of nanoscale sensor devices that can detect the presence of CO and water molecules. To overcome the reliability problem, these devices are developed by substitutional doping of impurity atoms (such as boron, nitrogen atoms) into intrinsic single-wall carbon nanotubes or by using composite B xCyNz nanotubes. Using first-principle calculations, we demonstrate that these sensor devices can not only detect the presence of CO and water molecules, but also the sensitivity of these devices can be controlled by the doping level of impurity atoms in a nanotube.
Carbon nanotubes have been demonstrated to be promising nanoscale molecular sensors for detecting gas molecules with fast response time and high sensitivity at room temperature.1,2 Semiconducting carbon nanotubes are shown to have sensitive conductance change in the presence of ppm concentration of gas molecules. The sensing mechanism is to detect the conductance change of the semiconducting single-wall carbon nanotube (S-SWCNT) induced by charge transfer from gas molecules adsorbed on nanotube surfaces. So far a few gas molecules (such as NO2,1 NH3,1 and O22) are shown to be detectable by these devices. However, the range of molecules that can be detected by nanotube sensors is limited to these molecules due to the electronic structure and chemical properties of intrinsic semiconducting carbon nanotubes.3-9 Intrinsic CNTs cannot detect a wide range of molecules, such as important toxic gases (e.g., carbon monoxide), water molecules, H2, and biomolecules since they do not adsorb on the nanotube surface. To overcome these limitations of intrinsic CNTs, diverse external or internal functionalization schemes are recently proposed.10,11 Even though these recent works have shown that these functionalized CNTs achieve certain degree of success on detecting molecular species that are not detectable to intrinsic CNTs (e.g., H2), it also raises a serious reliability issue. The existing efforts on externally functionalized CNT surface and internal doping in CNT result in only temporary sensing capability due to the weak van der Waals interaction between CNT and doped materials. It is a challenging issue to detect all kinds of important molecular species while maintaining many 10.1021/nl034064u CCC: $25.00 Published on Web 03/15/2003
© 2003 American Chemical Society
advantages of intrinsic carbon nanotubes as molecular sensors. Therefore detailed investigation is required to elucidate microscopic sensing mechanisms that can lead to a novel design of CNT sensors with a wide range of molecular sensing capacity and reliability. Here we propose the design of a new type of nanoscale molecular sensors using carbon nanotubes with modified electronical and chemical properties through doping impurity atoms into them. We demonstrate using ab initio simulations that this type of sensors can overcome the weaknesses of existing intrinsic and functionalized carbon nanotube sensors. Our design involves choosing different impurity atoms to be doped into intrinsic carbon nanotubes, and the impurity atoms are shown to be able to detect different molecular species. Two representative chemical speciesscarbon monoxide and water molecules, which are not detectable to either intrinsic or functionalized CNTssare studied to demonstrate the capability of this new type of sensors. The ab initio calculations are performed with the VASP12 using the density functional theory (DFT)13 and the local density approximation (LDA) with ultrasoft pseudopotential, plane-wave basis sets, and periodic boundary conditions. Local spin density approximation (LSDA) has also been tested to compare the accuracy of LDA calculations, where the differences between LDA and LSDA for reference energy and binding energy are within the error limit of DFT calculations ((0.05-0.10 eV). For nanotubes, the Kohn-Sham single electron wave functions are expanded by plane waves in a super cell 12.00 × 12.00 × 8.58 Å3
Table 1. Calculated Data for Adsorption of CO on Doped Carbon Nanotubes or BxCyNz Nanotubes
Figure 1. Schematic structure of doped nanotubes as a chemical sensor array to detect molecules such as carbon monoxide and water molecules.
corresponding to 495 eV cutoff energy. The Brillouin zone sampling is approximated by two k-points along the tube axis, which is shown to be a good approximation for both (8,0) and (10,0) nanotubes.14 The structural configurations of the isolated nanotubes are optimized through fully relaxing the atomic structures and the super cell sizes to minimize their total energies. The configurations of molecule-nanotube systems that are put in the super cell with the same size and the same k-point samplings are optimized through fully relaxing the atomic structures until the remaining forces are less than 0.1 eV/Å. Figure 1 depicts a schematic structure of this new type of devices that can detect the presence of certain gases, such as carbon monoxide and water molecules. We discover that boron- or nitrogen-doped carbon nanotubes experience a drastic change in electrical properties when exposed to carbon monoxide or water molecules. When an intrinsic carbon nanotube is doped with either boron atoms or nitrogen atoms through a replacement of carbon atoms, the local physical properties around the impurity atoms (boron or nitrogen) undergo a significant change, resulting in the change of the local chemical reactivity. This local chemical reactivity change15 increases the binding energy and the possibility of gas molecules interacting with the doped nanotube. Gas molecules bound on the doped CNTs can be detected through a charge transfer between the molecules and CNT that induces a conductance change of the CNT.1,2 The charge transfer is driven by the chemical potential realignment when the doped nanotube and gas molecules are put into one system. Furthermore, the charge-transfer capacities are different for the nanotubes doped with different impurity atoms. These findings lead us to the design of a wide range of nanotube sensor devices that are capable of detecting a variety of gas species by using different impurity atoms and their complexes as doping agents. The sensitivity of these devices to the presence of carbon monoxide molecules, as compared to that of the devices using intrinsic carbon nanotubes, is given in Table 1. All the configurations studied for carbon monoxide molecules are shown in Figure 2. To determine the conductance changes through the nanotubes, charge-transfer calculations were carried out using the local atomic sphere approximation that charge density of a molecule is distributed within the volume of atomic spheres. When the molecule is adsorbed on the nanotube surface, some charge would be transferred between 514
Eg (eV)c
configuration
CNT
0.56
B-CNTa
0.44
N-CNTb
0.43
BC3
0.61
COe OCf CO OC CO OC CO OC
Eb (eV)
D (Å)
ET (el)d
-0.85 -0.27 -0.22 -0.18 -1.72 -0.28
no binding no binding 1.52 2.83 2.99 2.82 1.50 2.53
0.59 0.02 0.05 0.08 0.50 0.04
a Intrinsic nanotube doped with boron atom in 2-unit cell. b Intrinsic nanotube doped with nitrogen atom in 2-unit cell. c HOMO-LUMO band gap. d Electron charge transfer from nanotube to molecules. e CO molecule binds to the nanotube with the carbon atom close to the tube. f CO molecule binds to the nanotube with the oxygen atom close to the tube.
Figure 2. Semiconducting (8,0) CNT with one boron atom substituition that binds to CO molecule. (A1) Boron atom is close to the carbon atom of a CO molecule. (A2) Boron atom is close to the oxygen atom of a CO molecule; semiconducting (8,0) CNT with one nitrogen atom substituition that binds to CO molecule. (B1) Nitrogen atom is close to the carbon atom of a CO molecule. (B2) Nitrogen atom is close to the oxygen atom of a CO molecule; semiconducting (8,0) BC3 nanotube that binds to CO molecule. (C1) Boron atom is close to the carbon atom of a CO molecule. (C2) Boron atom is close to the oxygen atom of a CO molecule.
the molecule and the nanotube, resulting in local charge changes within the atomic spheres of the molecule. The resulting charge transfer is calculated through the charge difference between the molecule adsorbed on the nanotube Nano Lett., Vol. 3, No. 4, 2003
Table 2. Calculated Data for Adsorption of H2O on the Doped Carbon Nanotubes or BxCyNz Nanotubes Eg (eV) f configuration Eb (eV) D (Å) ET (el)g CNT B-CNTa N-CNTb BN-CNTc
0.56 0.44 0.43 0.48
B2N-CNTd BN2-CNTe BC2N-type 2
0.44 0.42 0.93
H2O B-H2Oh N-H2Oi B-H2O N-H2O N-H2O B-H2O B-H2O N-H2O
-0.56 -0.23 -0.48 -0.24 -0.64 -0.53 -0.48 -0.18
no binding 1.70 -0.12 3.12 -0.02 1.72 -0.20 3.14 -0.02 3.15 -0.28 1.51 -0.08 1.72 -0.21 3.14 -0.05
a Intrinsic nanotube doped with boron atom in 2-unit cell. b Intrinsic nanotube doped with nitrogen atom in 2-unit cell. c Intrinsic nanotube doped with boron-nitrogen atom pair in 2-unit cell. d Intrinsic nanotube doped with boron-nitrogen-boron atom pair in 2-unit cell. e Intrinsic nanotube doped with nitrogen-boron-nitrogen atom pair in 2-unit cell. f HOMOLUMO band gap. g Electron charge transfer from the nanotube to molecules. h H O molecule binds to the nanotube with the oxygen atom close to the 2 boron atom. i H2O molecule binds to the nanotube with the oxygen atom close to the nitrogen atom.
and an isolate molecule itself.1,2 From Table 1, we can conclude that carbon nanotube with boron doping and intrinsic BC3 nanotube16,17 serve as good sensor devices for carbon monoxide due to the strong binding between them and a large electron charge transfer from a nanotube to carbon monoxide molecule, which leads to a large change in nanotube conductance. Table 2 summarizes the sensitivity of these devices to the presence of water molecules, as compared to that of the devices using intrinsic carbon nanotubes. All the configurations studied for water molecules are shown in Figure 3. From the results, we can conclude that carbon nanotubes with boron doping and intrinsic BC2N type 2 nanotubes18,19 serve as good sensor devices for water molecules due to the strong interaction between the molecules and the nanotubes, and a large electron charge transfer from molecules to nanotubes that drastically change the conductance of the nanotubes. The resulting charge transfer between the molecules and the nanotube is related to the gate capacitance of the devices and the diameter and the length of the nanotube: ∆Q ) Cg∆Vg ) δθ(πdl/σ). Cg is the capacitance of the nanotubes, and ∆Vg is the voltage change measured in the experiments, d and l are nanotube diameter and length respectively, while σ is molecule cross-section area and θ represents the molecule coverage on the nanotube surface. Additionally, a simultaneous doping of boron and nitrogen atoms into a carbon nanotube can lead to BmNn defect complexes. This possibility of defect complex engineering will increase the sensitivity of the doped nanotubes and thus make them very promising nanosensors due to the strong binding energy between doped nanotube and molecule species. Comparing the binding energy and charge-transfer resulted for the intrinsic nanotube detecting NO2,1 we can conclude that the B-doped nanotube will give under ppm level sensitivity for CO and H2O due to its similar binding energy and similar amount of charge-transfer per molecule. The binding energies of CO and H2O molecules with the boron-doped CNTs indicate that these molecules undergo Nano Lett., Vol. 3, No. 4, 2003
Figure 3. (A1) Semiconducting (8,0) CNT with one carbon atom replaced by a boron atom that binds to an H2O molecule, where the boron atom is close to the oxygen atom. (A2) Semiconducting (8,0) CNT with one carbon atom replaced by one Nitrogen atom that binds to an H2O molecule, where the nitrogen atom is close to the oxygen atom. Semiconducting (8,0) CNT with one carbon atom replaced by a boron atom and another by a nitrogen atom that binds to an H2O molecule, where the boron atom is close to the oxygen atom as in (B1), and the nitrogen atom is close to the oxygen atom as in (B2). (C1) Semiconducting (8,0) CNT with two carbon atoms replaced by two boron atoms and one by a nitrogen atom that binds to an H2O molecule, where the nitrogen atom is close to the oxygen atom. (C2) Semiconducting (8,0) CNT with one carbon atom replaced by one boron atom and two others by two nitrogen atoms that bind to an H2O molecule, where the boron atom is close to the oxygen atom. Semiconducting (8,0) BC2N type 2 nanotube that binds to an H2O molecule, where (D1) BN2 group binds to an H2O molecule with the boron atom close to the oxygen atom, and (D2) B2N group binds to an H2O molecule with the nitrogen atom close to the oxygen atom.
chemical adsorption, while physical adsorption occurred when CO or H2O molecules adsorbed on the nitrogen-doped CNTs. Reversing the adsorption of these molecules can be realized through applying UV light to the doped nanotube 515
to desorb the molecules from the tube surface.20 Boron-doped CNTs show stronger binding energies and shorter binding distances for both CO and H2O sensing cases. The stronger binding energies and shorter binding distances are consistent with the larger charge transfer between molecules and doped CNT systems. This indicates that the binding characteristic between molecules and doped CNT system is an ionic type. And the electron charge transfer is an important mechanism in changing the conductivity in the doped CNTs when adsorbed with CO or H2O molecules. To develop a rational understanding on the chemical reactivity of doped CNTs, we have examined the local softness of CNTs. The mechanism behind the charge transfer between a doped CNT and gas can be viewed through shortrange interactions associated with close contact between doped atoms and gas molecules. The close-contact chargetransfer capabilities of doped CNT are defined by the local chemical softness,21 s(r) )
( ) ∂F(b) r ∂µ
T,V(b) r
≈
1 ∆µ
r ∫µµ + ∆µ dEg(E,b),
where F(r) is the electron density, µ is the chemical potential, T is the temperature, ν(r) is the external potential, and g(E,r) is the local electron density of states. The approximation represented by the integral is an infinitesimal difference scheme that estimates local softness using the “frontier bands” of the CNT or doped CNT, and the quantity ∆µ is related to the width of the frontier bands of the system. Here, s+(r) and s-(r) describe the two kinds of softness: s+(r) characterizes the electron charge-transfer capacity from the CNT (or doped CNT) to the target molecules and s-(r) measures the charge acceptance capacity of the CNT (or doped CNT) from the molecules. The variation of the local softness on the CNT (or doped CNT) surface permits us to identify initial molecular adsorption sites during a reaction process according to the principle of local hardness, softness, and acid base (HSAB). The local softness provides us with an idea of the preferred location to induce a charge-transfer process. Figure 4 shows that the donor capacity s+(r) and the acceptor capacity s-(r) are uniform for all the carbon atoms within an intrinsic CNT. However, when a boron atom is introduced into a CNT system, the donor capacity becomes larger around the boron atom hexagonal ring while the overall donor capacity of carbon atoms far away from the boron atoms is reduced. Similarly, the capacity for the boron-doped CNT to accept the electron charge also becomes larger around the boron atoms, but the changes in acceptance capability are, however, localized around the boron atom within neighboring hexagonal rings. The local softness (s+(r), s-(r)) of N-doped CNT show similar changes to those of B-doped CNT, but the magnitudes are smaller, in good agreement with the weaker binding and smaller charge transfer as shown in Tables 1 and 2. This analysis clearly shows that a doped CNT increases the charge-transfer capability as compared to that of an intrinsic CNT. This quantitative understanding on reactivity changes of doped CNTs will enable us to develop a rational design of defect complexes that can detect a wide range of molecular species. 516
Figure 4. Local softness for an intrinsic CNT, a B-doped CNT and a N-doped CNT. (A1) s+(r), (A2) s-(r), for the intrinsic CNT. (B1) s+(r), (B2) s-(r), for the boron-doped CNT (red dotted curve indicates boron atom). (C1) s+(r), (C2) s-(r), for the nitrogen doped CNT (red dotted curve indicates nitrogen atom).
Experimentalists already have demonstrated the growth of B- or/and N-doped CNTs and BxCyNz nanotubes.16 BxCyNz and B-doped multiwall and single wall nanotubes have been synthesized by a carbon nanotube substitution reaction in which carbon atoms of starting CNTs have been substituted partially or totally by boron and/or nitrogen atoms by reaction with B2O3 with CNTs under Ar or N2 atompephere.22 This suggests that B- and/or N-doped CNT as gas sensor is experimentally realizable. By controlling the doping process during nanotube growth, one can form diverse defect complexes at different positions on the nanotube surface. This controlled doping will make the doped CNTs very flexible to detect wide range of molecular species and also very selective to specific molecules. The selectivity comes from the control of doping different atoms and complexes into nanotube array as shown in Figure 1. The engineered array of nanotubes will have unique sensitivity signatures of target molecule species. In conclusion, doped CNTs provide a welldefined approach to design and engineer nanotube sensor materials with high sensitivity, reliability, flexibility, and selectivity. Acknowledgment. This work was supported by NSF & SGF. The calculations are performed on Origin 3800 through the “Nanoscale Materials Simulations”. Nano Lett., Vol. 3, No. 4, 2003
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